Organic light emitting diodes (OLEDS) are electro-luminescent (EL) devices for emitting light. The OLED generates light by a current flowing through an organic compound. Pixels including the OLEDs have various advantages, i.e. simple structure, fast response and wide viewing angle. There are two types of matrix displays with the OLEDs, passive type and active type. In the active matrix display, thin-film transistors (TFT) are provided in each pixel to drive the OLEDs of display. The active matrix eliminates high peak driving currents and thereby enables high-resolution and high information density, improves power consumption and life-time compared to the passive matrix.
Vertical pixel architecture, in which the TFT and the OLED device are stacked vertically, has been developed. Such architecture can achieve higher aperture ratios. This favors using lower mobility amorphous silicon TFT backplanes compared polysilicon TFT technology, which is of higher mobility but also of higher cost.
The difficult part in building the vertical stacked pixels is to make a TFT backplane suitable for subsequent OLED fabrication and provide high yield and good performance of OLED pixels. The OLED device is typically made of very thin layers. Overall thickness of organic layers in the OLED is of the order of 100 nm. For this reason, it requires a smooth substrate to achieve good performance and yield. Step-wise features on the substrate surface and roughness can cause deterioration of light-emitting properties or OLED device failure due to shorts between its electrodes.
It is, therefore, desirable to provide new pixel architecture, which can achieve a high aperture rate, and at the same time, higher yield rate.